[Dovecot] POP3 vs. IMAP Load/Memory usage in Dovecot 1.0.15

[lists at truthisfreedom.org.uk]

Hi Stan,

Quoting Stan Hoeppner <stan at hardwarefreak.com>:

> On 7/11/2011 4:28 AM, lists at truthisfreedom.org.uk wrote:
>> Quoting Stan Hoeppner <stan at hardwarefreak.com>:
> This still doesn't provide us with the necessary information to give you
> an intelligent answer to your question.

Sorry, I thought I'd given quite a large amount of detail so far.

To answer the questions I believe were in your analogy:

* All the servers are made by the same manufacturer (Dell)
* They are all the same model (R410)
* The have the same engine (24 cores, 24G RAM, SAS Drives)
* The motorway is exactly the same for all servers (NFS to a NetApp  
6080 and a RAMSAN)
* The weather is almost exactly the same (Same Datacentre, different  
rooms/racks)
* The Driver is exactly the same (Dovecot 1.0.15)

>> The vast majority of the RAM usage is cache, however there is still a
>> discrepancy between the IMAP servers and the POP3 servers.
>
> A discrepancy where?  RAM usage by the pop and imap processes?  Is there
> any reason why you didn't post the actual data?

I thought I had explained this, but obviously not.

The discrepancies lie in two areas:

1) Load Average
2) RAM Usage (particularly in regard to cache)

In both cases, the value for each area is higher on the three nodes  
running POP3 than the nodes running IMAP.

>> I guess all I'm really after knowing is if there is a reason why this is
>> the case so I can put my mind (and those of my team!) at ease before we
>> start making other changes to the infrastructure - the last thing I want
>> to do is increase the load on these nodes and watch them die because
>> they didn't have enough resources.
>
> You still have not demonstrated what resources, if any, these nodes are
> lacking.  The only thing you have mentioned is memory consumption.  All
> Unices today will dump cache pages if a process needs memory space and
> will instantly reallocate it.  If the bulk of the RAM on these systems
> is consumed by disk cache, you don't have a problem.  If the "load" you
> mentioned is caused by something other then memory usage, then can you
> please show detail of such?  Could you at least provide a snapshot of
> top output from one pop and one imap machine?

POP3: https://gist.github.com/1075816
IMAP: https://gist.github.com/1075821

Unfortunately I can't provide access to the Munin Graphs owing to  
company policies, however I'm happy to post the output of pretty much  
any command (except `rm -rf` ;) ) that you would like to see.

I hope that's enough detail, if not please let me know.

Thanks again,

Matt